Function to rotate a point around another point

Question

so I've got a math function here that supposed to return a rotated point, and takes an original point, point to rotate around (origin) and radians to rotate it.

However it rotating only at half speed (aka 180 degree movement = 90 degree rotation)

sf::Vector2f RotatePoint(sf::Vector2f origin, sf::Vector2f point, float radian) {   
    float s = sin(radian);   
    float c = cos(radian);  

    // translate point back to origin:  
    point.x -= origin.x;   
    point.y -= origin.y;   

    // rotate point   
    float xnew = point.x * c - point.y * s;   
    float ynew = point.x * s + point.y * c; 

    // translate point back to global coords:
    sf::Vector2f TranslatedPoint;
    TranslatedPoint.x = xnew + origin.x;  
    TranslatedPoint.y = ynew + origin.y; 

    return TranslatedPoint;
}

Can you post up your calling code, too, with the expected and actual results? I believe the code works, have you calculated the radian measure properly? — evgeny

janneb janneb · Accepted Answer · 2011-09-16T06:44:14

The function looks ok to me. The rotation, in case you're wondering, is just multiplying a vector by the 2D Euclidean rotation matrix (http://en.wikipedia.org/wiki/Rotation_matrix ). The only errors I can think of are some misunderstanding of the usage of the function. E.g. that 2*PI radians = 360 degrees, or that the rotation is counterclockwise.

Function to rotate a point around another point

3 Answers